Communications: radio wave antennas – Antennas – Spiral or helical type
Reexamination Certificate
1999-01-25
2001-01-30
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
Reexamination Certificate
active
06181295
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is that of wide passband antennas with hemispherical or quasi-hemispherical radiation patterns. More specifically, the invention relates to resonant helix antennas and especially to the power supply for said antennas.
The antenna of the invention can find application especially in mobile satellite communications between users in fixed positions and moving bodies of all kinds for example, aeronautical, maritime or land-based bodies. In this field, several satellite communication systems are being implemented or are currently being developed (these include, for example, the INMARSAT, INMARSAT-M, GLOBALSTAR, and other systems). These antennas are also valuable in the deployment of personal communications systems (PCS) using geostationary satellites.
For all these systems, which provide for links with geostationary satellites, the very great difference in incidence between the signals received or transmitted requires that the antenna should have a radiation pattern with hemispherical coverage. Furthermore, the polarisation has to be circular with a ratio of ellipticity of more than 5 dB in the useful band.
More generally, the invention can be applied in all systems requiring the use of a wide band, a radiation pattern with hemispherical coverage, circular polarisation and a good ratio of ellipticity.
In the above-mentioned fields of application, the antennas must have the above-mentioned characteristics either in a very wide passband in the range of 10% or in two neighbouring sub-bands respectively corresponding to reception and transmission.
The patent FR-89 14952 filed on behalf of the present Applicant has already described a known type of antenna particularly suited to such applications.
This antenna, called a resonant quadrifilar helix (RQH) antenna has characteristics very close to the criteria laid down in a frequency band generally limited to 5% owing to problems of impedance matching. Wider band operation is possible by using dual-layer RQH antennas. These antennas are formed by the concentric “nesting” of two electromagnetically coupled coaxial resonant quadrifilar helices.
A quadrifilar antenna is formed by four radiating strands. An exemplary embodiment is described in detail in A. Sharaiha and C. Terret, “Analysis of quadrifilar resonant helical antenna for mobile communications”, IEEE—Proceedings H, Vol. 140, No. 4, August 1993.
In this structure, the radiating strands are imprinted on a thin dielectric substrate and then wound on a cylindrical medium that is radio electrically transparent. The four strands of the helix are open or short-circuited at one end and electrically connected at the other end with conductive segments positioned on the base of the lower part of the supporting cylinder. The four strands of the helix are therefore excited through these conductive segments.
This antenna conventionally requires a supply circuit that excites the different antenna strands by signals having the same amplitude in phase quadrature. There are several known techniques used to obtain a supply circuit of this kind.
In the above-mentioned document “Analysis of quadrifilar resonant helical antenna for mobile communications”, this function is fulfilled by means of a structure using couplers (3 dB, −90°) and a hybrid ring. This assembly is implanted on a printed circuit placed at the base of the antenna.
This technique has the advantage of being relatively simple to make and implement. By contrast, it leads to a non-negligible space requirement as compared with the antenna (which for example may have a size of about ten centimetres). This drawback makes this approach incompatible with many applications, especially when maximum miniaturisation is required.
According to a second technique described in J. L. Wong and H. E. King, “UHF satellite array nulls adjacent signals” (Microwaves & RF, March 1984), each bifilar helix may be supplied by a “folded balun” type of coaxial symmetrizer. The two bifilars helices are then excited in phase quadrature by means of a hybrid coupler.
The advantage of this method is that it requires the use of only one external hybrid element. By contrast, the symmetrizer/adapter assembly used for this type of antenna (made for example out of a coaxial section whose core and sheath form a dipole) is complex and bulky.
Furthermore, this type of assembly has the drawback of forming a sort of passband filter with a band that is still excessively narrow.
A third, more complex technique is described in C. C. Kilgus, “Resonant quadrifilar helix” (Microwave Journal, December 1970). The coaxial supply line is split at its end to form a symmetrizer. The phase quadrature is provided by adjusting the length of the strand.
This technique is used to eliminate hybrid couplers. However, it has the drawback of requiring a delicate adjustment of the length of the strand. Furthermore, the antenna is no longer symmetrical and the structure will be more complex. Besides, this method remains specifically reserved for systems using a narrow working band.
SUMMARY OF THE INVENTION
The invention is aimed in particular at overcoming these different drawbacks of the prior art.
More specifically, an aim of the invention is to provide an antenna and its system of supply (hereinafter the term “antenna” covers the antenna proper and its system of supply) having a very wide operating frequency band, for example greater than 10%.
Another aim of the invention is to provide an antenna of this kind with a low cost price that is easy to manufacture on an industrial scale. In particular, the invention is aimed at providing an antenna of this kind that can be manufactured in a very small number of successive operations.
Another aim of the invention is to provide an antenna of this kind that does not require specific and complex setting operations.
Yet another aim of the invention is to provide an antenna of this kind (and especially the supply system of such an antenna) taking up little space as compared with known devices.
The invention is also aimed at providing an antenna of this kind, achieving an equal-amplitude excitation of the four strands and a precise phase quadrature relationship and hence high quality circular polarisation in both sub-bands.
These aims as well as others that shall appear hereinafter are achieved according to the invention by means of a resonant helix antenna comprising at least one helix formed by at least two radiating strands printed on a substrate, comprising a miniaturised structure, for the wideband supply of said radiating strands, that is printed on said substrate and comprises at least one hybrid coupler made out of semi-localised (or “non uniformly spaced”) elements so as to reduce the dimensions thereof.
The making of the antenna strands and of the supply structure in the form of printed elements enables the the antenna, its supply structure and the duplexer to be made in only one operation without any specific connection means and in a particularly small format.
The use of hybrid couplers made out of semi-localised elements can be used to obtain all the desired features, and especially low space requirements as compared with systems based on the use of conventional lines.
The invention can be applied to all types of helix antennas. According to a preferred embodiment, said helix is a quadrifilar helix, formed by four radiating strands supplied by a supply structure comprising three hybrid couplers.
Advantageously, in the last-named case, said supply structure comprises a first 180° hybrid coupler associating a supply input and/or output of said antenna with two intermediate outputs and/or inputs phase-shifted by 180° and two 90° hybrid couplers each associating one of said intermediate outputs and/or inputs of said hybrid coupler with one of the ends of two of said radiating strands.
According to an advantageous embodiment of the invention, said antenna is mounted on a support having a first part and a second part that are distinct, with different values of permittivity, said first part beati
Blot Jean-pierre
Sharaiha Ala
Terret Claude
Toureilles Jean-Marc
France Telecom
Ho Tan
Kinney & Lange , P.A.
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